a) Field of the Invention
The present invention relates to a hydrodynamic bearing apparatus in which a shaft and a bearing are relatively and rotatably supported by hydrodynamic pressure of a bearing fluid. The present invention further relates to a method for making a hydrodynamic bearing apparatus.
b) Description of the Related Art
Conventionally, a hydrodynamic bearing apparatus is known in which a shaft and a bearing are relatively and rotatably supported by hydrodynamic pressure of a bearing fluid. In such a hydrodynamic bearing apparatus, if a bearing is made of a relatively soft material, such as copper alloy, to facilitate machining in the process of manufacturing, a problem occurs in the following way. When a bearing apparatus made of copper alloy and the like is applied in a motor of high-speed spins, a bearing gap becomes wider at a high temperature and narrower at a low temperature as the environmental temperature changes due to the high-speed spin, thus changing the bearing fluid viscosity required for generating hydrodynamic pressure. This degrades the hydrodynamic bearing property, that is, a motor property, thus making it previously impossible to make the bearing of a relatively soft material, e.g., a copper alloy. For this reason, in order to obtain a desired motor property, the bearing has been made of a metallic material that has low thermal expansion, e.g., stainless steel, ferrous material, or Ni-alloy. These materials are not easily affected by thermal expansion caused by changes in environmental temperature as compared to bearings made of a copper alloy.
Typically, a bearing is produced as follows. A bearing blank is initially produced from a pure material, the bearing blank is processed by rough machining and then rough finishing so as to produce a hole in the center on an inner circle surface of the bearing blank. The resultant bearing is machined to create an oil reservoir between the bearing portions, machined to cut hydrodynamic pressure generating grooves on the bearing portion, and bearing machined as the final machining step.
The above mentioned hydrodynamic bearing apparatus and manufacturing method, however, have the following problems. When a bearing member is made of metallic material having low thermal expansion, such as stainless steel, ferrous material, or Ni-alloy, and because these materials are generally difficult to machine (e.g., cut, roll, etc.), the precision of the machined hydrodynamic pressure generating grooves and the like (e.g., the precision of post-machining) is quite low. In addition, such machining takes a considerably long time. Still further, tools having a long tool life, e.g., diamond tools, cannot be used during the above-mentioned machining processes and, thus, tools having a relatively short life disadvantageously must be used.